This invention relates to a water-in-oil emulsion and more particularly to a water-in-oil emulsion which finds application in cosmetic products, quasi-drugs and other fields and which does not separate into water and oil phases but remains stable and provides a good skin feel.
As the emulsions in use as a cosmetic products and quasi-drugs, O/W (oil-in-water) milk lotions and creams or W/O (water-in-oil) milk lotions and creams are known.
O/W milk lotions and creams are satisfactory in the stability and feel of the system but rather poor in skin barrier property and absorbability of active ingredients. On the other hand, W/O milk lotions and creams are antithetical to said O/W milk lotions and creams in that while the former are satisfactory in skin barrier property and absorbability of active ingredients, they have the disadvantage that the system is unstable and liable to separate into aqueous and oil phases. Prevention of water-oil phase separation can be achieved by increasing the formulating amount of a solid oil basis but the practice aggravates the skin feel due to the stickiness and poor spreadability on the skin.
The inventors of this invention found that a water-in-oil emulsion improved in the above problematic attributes can be obtained by adding a liquid oil component gelling agent to a liquid oil basis to gelatinize the latter and mix-emulsifying it with water gelatinized with a specific water-in-oil type surfactant in advance.
The water-in-oil emulsion of this invention comprises liquid oil basis, liquid oil component gelling agent, water, and water-in-oil type surfactant which is polyglycerin fatty acid ester and/or polyethylene hydrogenated castor oil.
The water-in-oil emulsion of this invention includes milk lotions and creams, among others.
The liquid oil basis for use in this invention is an oil basis which is liquid at ambient temperature and, as such, includes but is not limited to fatty acid esters (e.g. cetyl isooctanoate, hexyl laurate, decyl oleate, methyl polysiloxane, cetyl octanoate, isopropyl myristate, isocetyl myristate, etc.), liquid paraffin, olive oil, tsubaki oil, rapeseed oil, 2-octyldodecanol, squalane, squalene, oleic acid and so on. These liquid oil bases can be used independently or in a combination of two or more species.
The formulating amount of the liquid oil basis in the composition of this invention is 5xcx9c30%, preferably 10xcx9c25%, by weight.
The liquid oil component gelling agent as used in this invention is intended to gelatinize said liquid oil basis and other optionally formulated oily component inclusive of the active ingredient, and includes, among others, aluminum hydroxide, 12-hydroxystearic acid, carnauba wax, microcrystalline wax, and straight-chain higher saturated fatty acid esters of dextrin (e.g. dextrin palmitate, dextrin stearate, dextrin behenate, dextrin myristate, dextrin cocofatty acid ester, dextrin laurate, etc.). Among these esters, dextrin palmitate is preferred.
The formulating amount of said gelling agent in the composition of this invention is 0.5xcx9c15%, preferably 1xcx9c10%, by weight.
Water for use in this invention may for example be purified water, distilled-water or tap water.
The proportion of water in the composition of this invention is 30xcx9c80%, preferably 40xcx9c70%, by weight. To insure a further improvement in skin feel, the formulating amount of water in the composition of this invention is preferably larger, particularly not less than twice larger, than the formulating amount of said oil component inclusive of said liquid oil basis.
The water-in-oil type surfactant to be used for gelling said water in this invention includes polyoxyethylene hydrogenated castor oil (e.g. polyoxyethylene (3)-hydrogenated castor oil, polyoxyethylene (5)-hydrogenated castor oil, polyoxyethylene (7)-hydrogenated castor oil, polyoxyethylene (10)-hydrogenated castor oil, etc.), polyglycerin fatty acid esters (e.g. polyglycerin condensed ricinoleate, polyglycerin stearate, polyglycerin oleate, etc.). With other kinds of water-in-oil type surfactants, the objective emulsifier composition which is stable and has a good skin feel cannot be obtained.
These water-in-oil type surfactants can be used independently or in a combination of two or more species.
The formulating amount of the water-in-oil type surfactant in the composition of this invention is 0.5xcx9c15%, preferably 1xcx9c10%, by weight.
Preferably the composition of this invention further contains one or more active ingredients. The active ingredients which can be incorporated include, among others, antipruritic agents such as diphenhydramine, diphenhydramine hydrochloride, diphenhydramine salicylate, crotamiton, chlorpheniramine maleate, etc., antiinflammatory agents such as glycyrrhetinic acid, dipotassium glycyrrhetinate, and emolients such as urea, sodium hyaluronate, and glycerin.
When a liquid oily drug (e.g. diphenhydramine, crotamiton, etc.) is to be formulated in a large amount, the formulating amount of said liquid oil basis may be reduced to bring the total amount of the liquid oil basis and oily drug into the above-mentioned formulation range.
The composition of this invention may be further supplemented with the conventional buffer (e.g. potassium dihydrogenphosphate, anhydrous sodium monohydrogenphosphate, etc.), a flavoring agent (e.g. 1-menthol etc.), an emulsifier stabilizer (eg. an inorganic salt such as magnesium sulfate), and an antiseptic (e.g. methyl p-hydroxybenzoate, propyl p-hydroxybenzoate, etc.).
The composition of this invention is produced as follows. On the one hand, said liquid oil component gelling agent, water-in-oil type surfactant, optionally liquid oily drug, flavoring agent, etc. to said liquid oil basis and the mixture is warmed for dissolution to give a homogeneous solution (A).
On the other hand, optionally said active ingredient (e.g. urea), buffer, antiseptic etc. are added to water and warmed for dissolution to give a homogeneous solution (B).
To the above solution (A) is added the above aqueous solution (B), and the mixture is stirred for emulsification, followed by cooling to ambient temperature.
Stability Test
The milk lotions obtained in Examples 1xcx9c4 and Comparative Examples 1xcx9c5, presented hereinafter and summarized in Table 2, were respectively examined grossly for the appearance immediately after preparation. The lotions were stored in a constant-temperature room at 40xc2x0 C. or 50xc2x0 C. and examined for the appearance after 6 months or 3 months, as the case may be, and evaluated according to the following criteria.
Water-oil phase separation:
Sensory test
In a panel consisting of 15 men and women from 30 to 75 years of age, a sensory test was performed for spreadability and stickiness on the skin.
Based on the sum of scores for n=15 for each attribute, the test result was expressed as follows.
Spreadability on skin:
xe2x89xa740 points: ∘
30xcx9c less than 40 points: xcex94
 less than 30 points: x
Tackiness on skin:
xe2x89xa740 points: ∘
30xcx9c less than 40 points: xcex94
 less than 30 points: x
The results are shown in Table 1.
It will be apparent from Table 1 that the composition of this invention does not separate into aqueous and oil phases but remains stable for a long time after production and that the composition is very satisfactory in skin feel as well.